!!The state of play of farmland birds by Hanna Kuzyo

Ibis (2004), 146 (Suppl. 2), 1–13

The state of play of farmland birds: population trends and conservation status of lowland farmland birds in the United Kingdom

Blackwell Publishing, Ltd.

RICHARD D. GREGORY, 1* DAVID, G. NOBLE 2 & JOHN CUSTANCE 3 The Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2DL, UK 2 British Trust for Ornithology, Thetford, Norfolk IP24 2PU, UK 3 Department for Environment, Food and Rural Affairs, 5/D12 Ashdown House, 123 Victoria Street, London SW1E 6DE, UK 1

Many bird species of lowland farmland have declined substantially in the United Kingdom over the past 30 years. Declines among farmland specialists are steeper than for generalists and were most rapid for these specialists in the 1970s and 1980s. These changes have been linked to increased agricultural intensification and are reflected in Red or Amber conservation concern status for many common farmland species, as well as for rarer ones. We review long- and short-term population trends and the conservation status of lowland farmland birds in the UK using the latest available information from bird surveys, and examine patterns among species. Analyses of demographic parameters suggest broadly that the key factor driving population changes of seed-eating and migrant birds is overwinter survival, whereas for many non-passerine species population growth appears to be limited by productivity. Population trends for a suite of lowland farmland species were first combined in the UK Government’s headline wild bird indicator published in 1998. This ‘Skylark index’ as it is sometimes known was intended to reflect the health of the wider countryside and struck a chord with the public and decision-makers. We look at the behaviour of the composite indicator and explore the population dynamics of the increasing and declining species separately. Simple models of population growth in these groups are then used to explore plausible scenarios for delivering the Government’s Public Service Agreement target to reverse the long-term decline in the number of farmland birds by 2020.

THE STATUS OF FARMLAND BIRDS Although a series of studies conducted in the 1980s and early 1990s had focused on problems facing birds in agricultural habitats (O’Connor & Shrubb 1986, Potts 1986, Galbraith 1988, Green & Stowe 1993, Evans & Smith 1994, Green & Griffiths 1994, Green et al. 1997, 2000), the scale of problems facing lowland farmland birds in the United Kingdom did not become fully apparent until the mid-1990s (Marchant et al. 1990, Gibbons et al. 1993, Fuller et al. 1995). Fuller et al. (1995) reviewed the status of a range of bird species and was able to show a

*Corresponding author. Email: richard.gregory@rspb.org.uk

preponderance of declines in geographical range and population size among lowland farmland birds. What was striking was not only the scale of the declines, but also the similarity in their pattern and timing across species. Subsequent analysis has confirmed and extended our understanding of the trend patterns (Siriwardena et al. 1998, Fewster et al. 2000). It is thanks to the existence of long-term bird monitoring schemes and bird atlases, both largely dependent on skilled volunteers, that we have such a detailed picture and understanding of farmland bird declines (Marchant et al. 1990, Gibbons et al. 1993, Fuller et al. 1995, Siriwardena et al. 1998, Aebischer et al. 2000a, Fewster et al. 2000). A raft of subsequent studies has explored the ecology and conservation of lowland farmland birds and there is compelling and extensive evidence to

R. D. Gregory, D. G. Noble & J. Custance

show that the declines among farmland birds in the UK, and elsewhere in north and west Europe, have been driven by agricultural intensification (Tucker & Heath 1994, Krebs et al. 1999, Aebischer et al. 2000a, Chamberlain et al. 2000, Donald et al. 2001). The plight of farmland birds as a group was highlighted further by the revision of the Red Data List in 1996, which was the first to place strongly declining, but still relatively widespread and common birds in the highest category of conservation concern (Gibbons et al. 1996). Around one-third of the birds on the resulting Red list were associated with farmland. Although some were rare or localized species that were already of high conservation concern such as Cirl Bunting Emberiza cirlus and Stone Curlew Burhinus oedicnemus, others such as Grey Partridge Perdix perdix, Skylark Alauda arvensis and Corn Bunting Miliaria calandra were widespread and new additions to the Red list. All of these species have been given high priority within the UK Government’s Biodiveristy Action Plan, which has acted as a stimulus to research and conservation action, and importantly has defined targets for species recovery. Subsequent revisions of the population status of the UK’s birds have confirmed the general pattern of decline in lowland farmland birds, with further additions to the Red list such as Starling Sturnus vulgaris and Yellowhammer Emberiza citrinella (Gregory et al. 2002). However, the adoption of a wildlife indicator based on populations of breeding birds (Anon. 1998, 1999, 2002) has more recently provided a very significant impetus and focus for farmland bird research. Here we focus on birds that are associated with lowland farmland in the UK, i.e. are to some degree dependent upon and/or have significant populations in this habitat (Gibbons et al. 1993, Gregory & Baillie 1998, Gregory 2000) rather than taking a broader view of all birds that occur on farmland, other farmland types and open habitats (e.g. Newton 2004). The farmland bird index The wild bird index, which is sometimes popularly known as the ‘Skylark index’, is one of the government’s 15 headline indicators of Quality of Life (Anon. 1998, 1999, 2002). These are the key indicators that the government is using to measure the UK’s progress towards sustainable development. They cover issues of everyday concern to people such as education, health, jobs, the environment, crime and housing. The bird indicator was developed to give a

broad summary of the general health of wildlife in the British countryside. Although covering most common species, it is particularly focused on two key habitats – ‘farmland’, which covers about three-quarters of the land area of UK, and ‘woodland’, which covers about another 10%. The purpose of the bird indicator was initially as a tool to raise awareness. It has been one of the more successful of the 15 headline indicators, proving to be resonant both with the public and the media, and generating regular media interest. Having become established, the indicator was then used to set targets and to monitor progress. The bird index demonstrates the potential power of indicators. It turns a large amount of scientific data into a simple, understandable and meaningful presentation. This in turn has raised awareness and acted as a spur for research and policy action. A brief history of the wild bird indicator The wild bird indicator was first developed between 1997 and 1998 based on ideas proposed by the Royal Society for the Protection of Birds (RSPB) and WWF in an expert advisory group set up by the then Department of Environment, Transport and the Regions (DETR). The intention was to develop and review wildlife indicators to underpin the 1999 UK Sustainable Development Strategy (Anon. 1998, 1999). This group identified a number of wildlife indicators reflecting conservation issues: the number and condition of Sites of Special Scientific Interest, the number and status of priority species and habitats covered by Biodiversity Action Plans, the proportions of species of threatened status, etc. However, a key gap was broader indicators reflecting the general health of common and widespread wildlife in the countryside. These were needed to complement those indicators measuring the state of species and habitats of particular conservation value. The bird index was developed to meet part of this need. The first ideas were based on the concept of combining series reflecting ‘distance from target’ for key species and habitats, at least exploring this idea for birds (Bibby 1999). The advantage of the distance to target approach was it potentially enabled an index to be constructed by combining different species of animals, plants, birds, etc., with measures of the state of different habitats. Disadvantages included the need for agreed targets, which in the past had proved extremely problematical, and the need for good

The state of play of farmland birds

quality long-term trend data for a broad range of species and habitats. These ideas were developed by the RSPB, British Trust for Ornithology (BTO) and the former DETR to create an index that reflects similar trends without requiring the need for targets for individual species. Birds were chosen for a number of reasons. First, they are regarded as good general indicators of the broad state of wildlife in the countryside, because they are wide-ranging in habitat distribution and tend to be at or near the top of the food chain (Furness & Greenwood 1993). Secondly, there are very good historical data for bird population changes since the early 1960s. Thirdly, birds have considerable public resonance and hence were thought to be a useful means for raising awareness for general biodiversity issues. During the course of development, trends for groupings of birds associated with a number of different habitats were examined, but two key groups, for farmland and woodland, were identified. These birds and their habitats are found across most of the UK countryside. For this reason, the bird indicator, which was first published in the Government’s consultation on the Headline indicators in 1998, focused on these two particular habitat types (Anon. 1998). It was based on common (i.e. species having greater than 500 breeding pairs in the UK) native bird species. Species were classified to habitat using Gibbons et al. (1993: their table 13). Rare breeding birds were excluded because they are mostly the focus of dedicated conservation action and concentrated at a relatively small number of sites, and not representative of the wider countryside. Following a positive response to the consultation, this indicator was formally adopted by the UK Government as one of its new 15 Headline indicators of sustainable development (Anon. 1999). Since then both the former Ministry of Agriculture, Fisheries and Food (now Department for Environment, Food and Rural Affairs – Defra) and the Forestry Commission have developed key targets and commitments to reverse the long-term decline in farmland and woodland birds by 2020, and progress reviews are published regularly (Anon. 2002). Specifically, Defra adopted a Public Service Agreement (PSA) target in 2002 to ‘Care for our living heritage and preserve natural diversity by reversing the longterm decline in the number of farmland birds by 2020, as measured annually against underlying trends’. The publication of UK indicators has also encouraged parallel development for individual countries of the UK and English regions (Anon. 2003a, 2003b).

An overview Here we review the population trends and conservation status of lowland farmland birds in the UK using the latest information available. We examine patterns among species and where possible identify the key demographic stage that is acting to limit population growth. Next, we explore the population dynamics of the subset of farmland species that form part of the UK Government’s headline wild bird indicator and look at the behaviour of the composite indicator itself. We look at the properties of the farmland indicator and the delivery plan for farmland bird recovery. Finally, we use a simple model of population growth to explore plausible scenarios of future change with reference to the Government’s target to reverse the long-term decline in the number of farmland birds by 2020. METHODS For further details on: trend information, see Marchant et al. (1990), Crick et al. (2004), Raven et al. (2003), Gregory et al. (2003a) and Eaton et al. (2004); population status, see Gibbons et al. (1996) and Gregory et al. (2002); and indicator methods, see Gregory et al. (1999), (2003b) and Freeman et al. (2001). Trend information comes mainly from two large-scale annual surveys – the BTO’s Common Birds Census, which ran from 1962 to 2000 (Marchant et al. 1990, Crick et al. 2004), and the BTO/Joint Nature Conservancy Committee/RSPB Breeding Bird Survey (BBS), which started in 1994 (Raven et al. 2003). In brief, the farmland bird indicator is the mean index created by combination of the constituent species’ trends on a geometric scale, so that a doubling of one species’ index is balanced by the halving of another (Gregory et al. 1999, 2003b). RESULTS Population trends and conservation status of farmland birds Population trends and conservation status of a wide range of species that live on lowland farmland are given in Table 1. Nine of the species in the farmland bird indicator are Red-listed, based on population declines in excess of 50% over the last 25 years (Gregory et al. 2002). Of the three species that declined most dramatically (> 85%), Corn Bunting and Grey Partridge have continued to decline steeply, whereas

R. D. Gregory, D. G. Noble & J. Custance

Tree Sparrows Passer montanus appear to be recovering – up by 55% since 1994. Of the five species experiencing long-term declines of between 50 and 85%, Turtle Dove Streptopelia turtur continues to decline steeply, three (Skylark, Yellowhammer and Starling) show moderate declines, and one (Linnet Carduelis cannabina) has been broadly stable since 1994. The Yellowhammer appears better able to exploit cereal crops than other seed-eaters and its decline started later than most, in the 1980s (Bradbury & Stoate 2000; Morris et al. 2001, 2002). The Reed Bunting Emberiza schoeniclus population has declined by nearly 50% over the last 30 years but has been stable since 1994. All three amber-listed species in the farmland bird indicator have shown alarmingly steep declines since 1994, 30% for Kestrel Falco tinnunculus, 18% for Lapwing Vanellus vanellus and 14% for Yellow Wagtail Motacilla flava. Although not included in the indicator because of the lack of annual count data, the UK distribution of Barn Owls Tyto alba declined between the two breeding bird atlases by 39% (Gibbons et al. 1993). The latest population estimate was around 4000 pairs in 1995–97, but comparison with earlier surveys is complicated by differences in the methods (Toms et al. 2001). A recently initiated annual monitoring programme should improve monitoring for this species. Seven species in the farmland bird indicator are increasing (Table 1). For Greenfinch Carduelis chloris and Goldfinch Carduelis carduelis, both longterm and short-term trends are positive, and

Whitethroats Sylvia communis have been increasing in numbers since the population crash in the late 1960s. Three species (Jackdaw Corvus monedula, Woodpigeon Columba palumbus and Stock Dove Columba oenas) have almost doubled in population size since 1970, and all three species, as well as Rook Corvus frugilegus (Marchant & Gregory 1999), continue to increase or are stable in number. The high profile of the farmland bird indicator focuses attention on its constituent 20 species described above, but population trends in other lowland farmland species are also of interest. Barn Swallow Hirundo rustica numbers have fluctuated over the past few decades but are now moderately increasing. Numbers of Carrion Crow Corvus corone and Magpie Pica pica, like most other corvids, have increased steadily since the 1970s. Four rare species – Quail Coturnix coturnix, Corncrake Crex crex, Cirl Bunting and Stone Curlew – are Red-listed on the basis of historical declines, but the latter three have increased considerably over the past decade in response to targeted conservation management (Aebischer et al. 2000b, Peach et al. 2001, Gregory et al. 2003a; Eaton et al. 2004). Numbers of Quail are notoriously variable in the UK and the species is difficult to census because it is so cryptic in the field. However, the Red-backed Shrike Lanius collurio, once a relatively widespread bird, is now virtually extinct in the UK. One introduced farmland species – the Red-legged Partridge Alectoris rufa – has declined over the long-term but the current trend is positive. Numbers of Little Owl Athene noctua and

Table 1. Population trends and conservation status of lowland farmland birds in the UK. Conservation concern listing = a measure of population status (Gregory et al. 2002). UKBAP = a priority species in the UK Biodiversity Action Plan. FBI = inclusion in the farmland bird indicator. Long-term trends are for 1970 – 2001 and short-term trends for 1994 – 2002 unless otherwise stated.

Species Kestrel Falco tinnunculus Red-legged Partridge Alectoris rufa1 Grey Partridge Perdix perdix Pheasant Phasianus colchicus1 Corncrake Crex crex Stone Curlew Burhinus oedicnemus Lapwing Vanellus vanellus Snipe Gallinago gallinago Curlew Numenius arquata Stock Dove Columba oenas Woodpigeon Columba palumbus Turtle Dove Streptopelia turtur Cuckoo Cuculus canorus

Conservation concern listing

UKBAP

FBI

Trend 1970 –2001 (%)

Trend 1994 –2002 (%)

Amber Green Red Green Red Red Amber Amber Amber Amber Green Red Amber

No No Yes No Yes Yes No No No No No Yes No

Yes No Yes No No No Yes No No Yes Yes Yes No

−372 −24 − 86 +452,3 +144 −155 − 412 − 646 − 492 +872,3 +877 −77 − 432

− 30 +21 −18 +16 +2713 +5414 −18 +52 −20 +15 +5 − 42 −25

The state of play of farmland birds

Table 1. Continued

Species Little Owl Athene noctua1 Skylark Alauda arvensis Swallow Hirundo rustica Meadow Pipit Anthus pratensis Yellow Wagtail Motacilla flava Pied Wagtail Motacilla alba Wren Troglodytes troglodytes Dunnock Prunella mudularis Robin Erithacus rubecula Blackbird Turdus merula Song Thrush Turdus philomelos Lesser Whitetrhoat Sylvia curruca Whitethroat Sylvia communis Spotted Flycatcher Muscicapa striata Great Tit Parus major Red-backed Shrike Lanius collurio Magpie Pica pica Jackdaw Corvus monedula Rook Corvus frugilegus Carrion Crow Corvus corone Starling Sturnus vulgaris House Sparrow Passer domesticus Tree Sparrow Passer montanus Chaffinch Fringilla coelebs Greenfinch Carduelis chloris Goldfinch Carduelis carduelis Linnet Carduelis cannabina Bullfinch Pyrrhula pyrrhula Yellowhammer Emberiza citrinella Cirl Bunting Emberiza cirlus Reed Bunting Emberiza schoeniclus Corn Bunting Miliaria calandra

Conservation concern listing

UKBAP

FBI

Trend 1970 –2001 (%)

Trend 1994 –2002 (%)

Green Red Amber Amber Amber Green Green Amber Green Green Red Green Green Red Green Red Green Green Green Green Red Red Red Green Green Green Red Red Red Red Red Red

No Yes No No No No No No No No Yes No No Yes No Yes No No No No No No Yes No No No Yes Yes No Yes Yes Yes

No Yes No No Yes No No No No No No No Yes No No No No Yes Yes No Yes No Yes No Yes Yes Yes No Yes No Yes Yes

−5 − 54 +11 −312 −597 +52 +548 −36 + 43 −19 −52 +2 −189 −82 +51 −9010 +109 +83 +4011 +84 − 662 − 657,12 − 94 2 +29 +22 +35 − 51 − 57 − 52 +11912 − 48 − 897

+26 −14 +10 −5 −14 +23 +14 +13 +16 +16 +13 −27 +30 −42 +19 Extinct?15 +2 +12 0 +15 −13 −7 +55 −5 +31 +18 −4 −26 −13 +54 +3 − 41

Data are derived from Common Bird Census (CBC) plots from 1966 up to 2000 (except where indicated) and Breeding Bird Survey (BBS) plots from 1994 to 2002. For long-term trends, counts were modelled using a full site by year log-linear Poisson regression model with post hoc smoothing of the annual indices. Reported long-term population changes are the differences in smoothed annual indices in joint CBC–BBS models from 1970 to 2001 – the year prior to the last available data, except for House Sparrow (from 1976). All figures refer to the UK. However, for species where there is evidence of substantial and statistically significant differences in trends within and outside England, the overall trends are based solely on the CBC prior to 1994 and solely on the BBS from 1974 to 2002. Caveats related to unrepresentative habitat coverage, small sample sizes or fluctuating populations are listed. Short-term trends were derived from counts on BBS squares analysed using a full site by year log-linear Poisson regression model, and cover the period 1994 – 2002. 1 Species introduced into the UK. 2 The trend during the period covered solely by the CBC (prior to 1994) may be unrepresentative of the UK due to geographical or habitatrelated bias towards populations with low densities. 3 Long-term trend may be biased by differences in BBS and CBC methodologies. 4 1970–2003, RSPB. 5 1970–2000, RSPB. 6 1974–1999 Common Birds Census, Gregory et al. (2002). 7 Small sample size during some part of the survey period. 8 The species shows large natural fluctuations from year to year. 9 The population has not recovered from the 1968/69 crash caused by severe drought on the African Sahel wintering grounds. 10 1973–77 to 1995– 99. Rare Breeding Birds Panel, Gregory et al. (2002). 11 1975–77 to 1996. Rookeries Census, Marchant and Gregory (1999). 12 1976–2001. 13 1997–2003, RSPB. 14 1995–2000, RSPB. 15 Effectively extinct as a breeding species; last confirmed breeding in 1999.

R. D. Gregory, D. G. Noble & J. Custance

Pheasant Phasianus colchicus (both introduced into the UK) have also been broadly stable over the long term and increasing in the short term (Table 1). A number of species occur widely on farmland but prefer other habitats. Highly abundant in hedgerows and wooded areas of farmland, numbers of Wren Troglodytes troglodytes, Dunnock Prunella modularis, Robin Erithacus rubecula, Great Tit Parus major and Chaffinch Fringilla coelebs show positive long-term and short-term trends (Table 1). The Lesser Whitethroat Sylvia curruca is also associated with hedgerows and although its long-term trend is stable, it has shown a sharp decline since 1994. After significant declines between the 1970s and 1990s, Blackbird Turdus merula and the Red-listed Song Thrush Turdus philomelos are currently showing signs of a recovery. However, the Red-listed Bullfinch Pyrrhula pyrrhula and Spotted Flycatcher Muscicapa striata are continuing to decline steeply. In addition to Lapwing, two other waders – Curlew Numenius arquata and Snipe Gallinago gallinago – breed widely on farmland, mainly in upland or lowland wet grassland habitats (Wilson et al. 2004). A recent survey revealed that Snipe declined by more than 60% and Curlew by about 40% on lowland wet grasslands in England and Wales since the early 1980s (Wilson et al. in press), although this habitat caters for only part of their populations. Curlew has declined in abundance since 1994 whereas Snipe appear to have increased following longer-term decline. Among birds of open countryside, Pied Wagtail Motacilla alba numbers have increased whereas Meadow Pipit Anthus pratensis shows a moderate decline. Although largely an urban bird, House Sparrows Passer domesticus are commonly found on farmland associated with buildings. This recently Red-listed species has declined steeply (65%) since 1976 and continues to experience a much-publicized fall in numbers. Dividing the farmland indicator species into the seven species that have increased since 1970, and the 12 that have shown major declines (following the PSA delivery plan), is instructive (Fig. 1). The increasing species (Fig. 1a) can be subdivided into those that have increased over the whole period (four species that have increased by around 2% per annum) and those that declined but have then shown some recovery (three species that have increased by around 3% per annum since 1985). The patterns among the declining group of species are more similar (Fig. 1b), although there are signs of flattening and even upturn in the trends for some species in recent years; overall declines range from 2 to 10%

per annum from moderate to severe decline among these species. Taking this group of lowland farmland birds as a whole (and it is not an exhaustive or definitive list), 58% of them have shown long-term declines (> 10%) and 32% short-term declines (> 10%: Table 1). Demographic causes of decline Two sources of information have helped to shed light on the key demographic processes in population changes in lowland farmland birds in the UK: (i) integrated population modelling of survey, nest record scheme and ringing scheme data, and (ii) intensive autecological studies. Survival, as opposed to productivity, is implicated as the key factor in the population declines of seed-eating passerines, such as Cirl Bunting (Evans & Smith 1994) Reed Bunting (Peach et al. 1999), House Sparrow and Goldfinch (Siriwardena et al. 1999, Hole et al. 2002). It is also believed to be a contributing factor in the declines of Corn Bunting (Donald & Evans 1994, Donald & Aebischer 1997), Yellowhammer (Kyrkos 1997) and Tree Sparrow (Crick et al. 2004). However, for Linnet, the trend appears to be driven by reduced breeding performance (Siriwardena et al. 2000). Survival is also considered the main factor in the decline in Starlings (Freeman et al. 2002). The role of survival on population trends for Skylark (another species that feeds on seeds during the winter) is not clear, but the shorter breeding season resulting from changes in cropping regimes is believed to have a major role (Chamberlain & Crick 1999, Donald & Vickery 2001). A reduction in the number of breeding attempts is also thought to be driving the decline in Turtle Doves (Browne & Aebischer 2001). Agricultural practices during the breeding season are known to have adverse effects on the breeding performance of Corn Buntings (Brickle et al. 2000), Grey Partridge (Potts 1986), Stone Curlew (Aebischer et al. 2000b), Lapwing (Shrub 1990) and Corncrake (Green & Stowe 1993). It should be noted that the number of nesting attempts made per season is not provided by the nest record scheme, and it may be a crucial variable for some species. Among increasing farmland species, improved breeding performance is thought to have driven increases in Stock Dove (O’Connor & Mead 1984) and Jackdaw (Gregory & Marchant 1996), whereas trends for long-distance migrants such as Common Whitethroat and Barn Swallow appear to be influenced most strongly by survival (Baillie & Peach 1992).

The state of play of farmland birds

Figure 1. Individual trends of the 20 farmland species included within the UK farmland bird indicator: a group of (a) seven ‘increasing’ and (b) 12 ‘declining’ species as defined within the PSA delivery plan. There are no annual data for Barn Owl.

R. D. Gregory, D. G. Noble & J. Custance

Patterns of trend in the farmland bird indicator The indicator shows separate trends for ‘all common species’, ‘woodland’ and ‘farmland’ species (Gregory et al. 1999, 2003b, Freeman et al. 2001; Fig. 2a). The farmland bird index has fallen by 42% from 1970 to 2002; specialist farmland birds have declined by 58%, while generalists have increased by 4% from 1970 to 2002 (Fig. 2b). There is, however, a good deal of variation in the pattern of trends when we

Figure 3. The farmland bird indicator with modelled rates of decline at 5% per annum from 1977 to 1988 and 1% per annum from 1988 to 2002.

categorize birds simply as generalist or specialist. Population declines among farmland birds were most marked from the mid-1970s to the 1990s with a suggestion of a lessening of declines in the last decade. The farmland indicator trend is well described by a simple model in which an initial increase up until 1977 is followed by a 5% and then a 1% per annum decrease (Fig. 3). This supports the view that the rate of decline has lessened. The statistical tests underlying the PSA targets reinforce this important conclusion (see Fig. 4 below). DISCUSSION The farmland bird indicator

Figure 2. The UK Government’s headline wildlife indicator – (a) populations of wild birds and (b) the farmland bird indicator split into generalist and specialist farmland species.

The wild bird indicator describes the trends of common species drawing on a range of surveys (Gregory et al. 1999, 2003b). It is a composite indicator, i.e. it is based on a ‘basket’ of species and reflects the average behaviour of the constituent species. The species are weighted equally, so that trends in, for example, Woodpigeon count the same as those in Corn Bunting. We have resisted weighting the indices by population size or conservation status because each would lead the indicator in a particular direction. The first would create an indicator dominated by the most abundant bird species, the second by those species judged to be most threatened and thus not by the full suite of species present in farmland. Neither approach would seem appropriate given the role of this indicator. To reiterate, the purpose of the indicator is as a barometer of broad-scale change in the countryside and as a surrogate for changes in other wildlife. Birds

The state of play of farmland birds

can be good indicators because they often reflect changes in the state of other animal and plant populations (Furness & Greenwood 1993). There is, for example, evidence to show that farmland bird declines are concomitant with those in insect and plant populations probably driven by a common factor, the intensification of agricultural production (Rands & Sotherton 1986; Donald 1998, Sotherton & Self 2000, Benton et al. 2002). Such indicators, however, must be used with care as a guide to changes in the wider environment. There are potentially a number of complex causes of changes in bird numbers and hence in the indicator, such as food, habitat condition, predators, disease and competition. Cold winter weather has a significant short-term impact on susceptible bird populations and thus on the indicator from year to year, but weather does not appear to drive the overall trend. There is a positive correlation between the annual change in the farmland index and the Central England Temperature in the preceding winter (r 2 = 0.42, P = 0.001). A small number of the species in the farmland index may also be affected by conditions on their migration and wintering grounds in Europe and Africa (Baillie & Peach 1992). Delivering farmland bird recovery The Government’s PSA target aims to ‘Care for our living heritage and preserve natural diversity … by reversing the long-term decline in the number of farmland birds by 2020, as measured annually against underlying trends’. With this target is a detailed delivery plan that defines how the target is measured and how it will be achieved. The target is measured against a modified version of the farmland bird index: the trend line is ‘smoothed’ to remove sampling error and short-term peaks and troughs due to weather (Freeman et al. 2001) and only data collected from within England are used – the PSA target was adopted by Defra, whose land management schemes and other policies apply only in England. The target is judged to have been achieved when the trend in the index and the associated lower confidence limit (using 95% CLs) are both positive. The smoothed version of the farmland bird index reveals the underlying trend (Fig. 4a) and by showing the proportional change in this index from year to year, we are able to measure progress (Fig. 4b). The recent upward trend in the index is encouraging (Fig. 4b), but there is some way to go before there is a statistically significant increase. The delivery plan recognizes that the statistical target is achievable in various ways, some of which might be

Figure 4. (a) A smoothed version of the farmland bird indicator for England with 95% confidence limits derived by bootstrap (dashed lines). (b) The smoothed English farmland bird index presented as a proportion of the previous year’s index with 95% CLs on the proportional change. Values > 1.0 indicate increase and < 1.0 indicate decrease between years.

judged undesirable. Meeting the statistical test alone would not be sufficient to have delivered the overall target. For example, the statistical test could in theory be met by increases among a few highly abundant generalist species, while specialists continued to be lost (see below). The vision within the plan is to have stable or rising populations of the 20 key farmland birds. The Government’s aim is to bring each species into a stable or increasing position and to safeguard and sustain that achievement. What does the future hold? Having described the historical pattern of population trends among farmland birds, we can also look forward. With a range of policy initiatives in place, there is room for optimism, but what sort of outcomes might we expect from what we know about farmland birds? We have looked at this question by modelling trends in the overall indicator and then by separately considering trends among the ‘increasing’ (n = 7) and ‘declining’ (n = 12) group of species in

R. D. Gregory, D. G. Noble & J. Custance

Figure 5. Modelling future scenarios of population change among the farmland birds within the indicator from 2002 (marked by an arrow) to 2020. (a) Overall change in the farmland indicator with average population increases of 1, 2, 3 and 4% per annum. In parts b–f, the ‘increasing’ species are represented by filled symbols, ‘declining’ species by open symbols, and farmland bird index by a line. (b) Increases of 1% per annum among all the ‘increasing’ (top line) and ‘decreasing’ species (bottom line) – the overall change in the farmland bird index is +20% 2002 – 2020 (middle line). (c) Increases of 3% per annum among all the ‘increasing’ (top line) and ‘decreasing’ species (bottom line) – the overall change in the farmland bird index is +70% 2002 – 2020 (middle line). (d) Stability among the ‘increasing’ species (top line) and increases of 1% per annum among the ‘decreasing’ group of species (bottom line) – the overall change in the farmland bird index is +12% 2002 – 2020 (middle line). (e) Stability among the ‘increasing’ species (top line) and increases of 1% per annum among nine and decreases of 3% per annum among three of the ‘decreasing’ species (bottom line) – the overall change in the farmland bird index is 0% 2002–2020 (middle line). (f) Increases of 3% per annum among the ‘increasing’ species (top line) and decreases of 1% per annum among the ‘decreasing’ group of species (bottom line) – the overall change in the farmland bird index is +9% 2002 – 2020 (middle line). © 2004 British Ornithologists’ Union, Ibis, 146 (Suppl. 2), 1–13

The state of play of farmland birds

the farmland bird index. This is a simple model with constant rates of change and no formal testing of targets, but the results are revealing. It is obvious that any positive and sustained average increase in the constituent trends of the farmland indicator will move us towards the PSA target (Fig. 5a). Interestingly, an average increase of 3% per annum, which is high but realistic, brings the farmland bird index back to its value in 1970 by 2020. Similarly, if we model the fate of the ‘increasing’ and ‘declining’ species separately, average increases of 1% and 3% per annum hold the potential of delivering the farmland bird target by 2020 (Fig. 5b,c), although this would need to be confirmed by examining the confidence limits around these trends. Even if modest increases of 1% per annum were restricted to the declining species group, there is still a realistic chance of approaching the farmland bird target by 2020 (Fig. 5d). It is, however, probably unrealistic to assume that each species will respond in the same positive way to policy mechanisms and other drivers, and if only a small number of declining species falter, the PSA target may not be met (Fig. 5e). Declines in as few as two or three species could jeopardize a positive outcome. A further kind of undesirable outcome would come about if population increases in the increasing group strongly outweighed population declines among the declining group. For example, the overall decline in the farmland bird index could be reversed by an upsurge in the ‘increasing’ species alone while the ‘declining’ species continued to fall in number (Fig. 5f ). This would not be judged sufficient to meet the PSA target, which aims to bring each species into a stable or increasing position. Our analysis shows that delivery of the PSA target will depend on the complex behaviour of the constituent species trends, and critically on reversing the trends among the suite of declining farmland species. We would like to thank David Gibbons, Mark Avery, Andy Evans, Juliet Vickery, Phil Grice, John Osmond, Rocky Harris, Mark Eaton, Richard Bradbury and Simon Wotton. We also thank the two referees for constructive comments.

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